During the combustion process, flue gases generated from furnaces contain nitrogen oxides (NOx). It is desirable to reduce NOx emissions into the atmosphere. One post-combustion process for the lowering of NOx emissions is that of selective catalytic reduction (SCR). Selective catalytic reduction systems use a catalyst and a reactant such as ammonia gas, NH3, to dissociate NOx to molecular nitrogen, N2, and water vapor. A utility steam generating power plant having, for example, a fossil fuel-fired furnace may utilize selective catalytic reduction (SCR) as a NOx reduction technique. The furnace generally comprises a furnace volume in fluid communication with a backpass volume. Combustion of hydrocarbon fuels occurs within the furnace volume creating hot flue gases that rise within the furnace volume giving up a portion of their energy to the working fluid of a thermodynamic steam cycle. The flue gases are then directed to and through the backpass volume wherein they give up additional energy to the working fluid. Upon exiting the backpass volume the flue gases are directed via a gas duct through a selective catalytic reduction chamber and thence to an air preheater and flue gas cleaning systems thence to the atmosphere via a stack.
In a SCR system, at some point in the gas duct after the flue gas stream exits the back pass volume and upstream of the SCR chamber, a reactant, possibly ammonia, in a gaseous form, or a urea/water solution is introduced into, and encouraged to mix with, the flue gas stream. The reactant/flue gas mixture then enters the SCR chamber wherein the NOx reduction takes place between the reactant and the flue gas mixture in the presence of the catalytic surfaces. The introduction of the ammonia or urea into the flue gas stream is generally achieved by the use of injector atomizing nozzles located at either the periphery of the gas duct, or immersed on injection lances within the flue gas stream.
While the SCR facilitates the reduction of NOx, sulfur trioxide (SO3) emissions are increased because the catalyst used for the NOx reduction, promotes oxidation of incoming sulfur dioxide (SO2) to sulfur trioxide (SO3). SO3 emissions at the stack must be limited to very low levels (below 5 ppm) to avoid excess opacity and/or a visible blue plume.
Downstream of the SCR, SO3 emissions are partially reduced by condensation in the combustion air preheater and captured in the particulate and SO2 control equipment. If this reduction proves insufficient, specific SO3 control measures are generally added to the process. One additional control measure is the addition of a spray dry absorber upstream of the flue gas desulfurization equipment (for removing both SO2 and SO3). Another is the addition of a condensing heat exchanger upstream of an electrostatic precipitator where the SO3 is captured by the fly ash/condensate. Other methods include dry or wet sorbent injection using ammonia, lime, sodium bicarbonate, trona, and the like. All of these methods add both capital and operating cost. It is therefore desirable to reduce the SO3 emissions to the atmosphere cost effectively with a minimum of capital expended on additional control equipment